1. Field of the Invention
The present invention relates to a dispensing device which is arranged to dispense an accurate and measured dose of a fluid. Typically such a fluid may be one which is itself or which contains a therapeutically or otherwise active compound or composition, such as insulin.
2. Description of the Related Art
European Patent Specification No. 0,037,696 A discloses a dispensing device suitable for use in dispensing a predetermined quantity of material from a container comprising a tubular body member having an outlet at one end, and a plunger slidably movable in said body member towards said outlet. The dispensing device comprises an elongate body having a chamber for receiving a container with its outlet held in a first end portion of the elongate body, and a first drive member mounted in said body for use in driving said plunger. In the disclosed device said first drive member is slidably mounted for driving engagement via a unidirectional drive transmission with a second drive member having a free end drivingly engagable with said plunger of the container. In that manner said second drive member and said plunger can be driven by the first drive member via said unidirectional drive transmission means only in a direction towards the container outlet and the first end portion of the elongate body whilst permitting return movement of the first drive member. Preferably, the unidirectional drive transmission comprises a ratchet means.
For setting the dose to be dispensed the adjust knob is turned to align a pin with one of a series of channels of varying length (dependent on the size of dose required) in the push button (or vice versa). The length of the channel dictates the movement forward of the plunger and the multiple of doses is decided by how many channels one can safely have around the circumference consistent with side walls to each channel and the size of the readout on the circumference.
A usable diameter gives eight variable doses. One could increase the diameter to give more channels and more variations of dose. However, the device would become more and more unwieldy as the number of channels is increased.
Another serious fault with this device is that it is easy to pump the push button so that the plunger moves forward without the pin ever reaching the bottom of its channel. This is unsafe.
European Patent Specification No. 0,295,075 A relates to a device for dispensing a fluid from a container by means of the axial movement of a piston within the container under the influence of a plunger moved by the device. The device is adapted to receive the container at its forward end and to move the plunger axially forward towards the container so as to dispense a selected amount of fluid from the container upon each actuation of the device. The device comprises a drive mechanism adapted to be reciprocated axially of the device and to be positively engaged with the plunger for the forward stroke of the drive mechanism so as to prevent relative movement between the plunger and the drive mechanism and to move the plunger forward. The drive mechanism requires a positive action to disengage it from the plunger so as to permit relative movement between the plunger and drive mechanism for at least rearward movement of the drive mechanism. Also, the forward travel of the drive mechanism is limited by a fixed stop mechanism and the extent of the forward stroke of the drive mechanism is individually selectable for each actuation of the device by withdrawing the drive mechanism or a part operatively associated therewith a selected distance from the said fixed stop.
The device of EP-0,295,075 A is designed to give a maximum of 36 units of insulin. For dose setting opposed jaws are pulled back in a slot by rotating a screw, the length of slot being determined by the length of the movement forward of the plunger at maximum dose. A larger dose means a longer slot and a corresponding lengthening of the device. In addition, to wind back the screw for an additional dose means a lengthening of the screw in the main body, thereby adding another additional dose length to the device. Also, in line with the jaws and screw is the readout which needs an additional dose length and a corresponding increase in body length. Thus, there is a ratio of 3:1 between body length and plunger length, which means that each time the dose is increased there is an additional three millimeters of body length for every millimeter of plunger length necessary to give the higher dose setting.
In prior art dispensing devices, such as those outlined above, the usual dispensing increment is 2 units, but the amount dispensed for a given standard length of device is limited. Thus, in the case of a patient wishing to use a relatively large dose it may be necessary to use the device twice or even more times, each time to dispense a smaller dose.